From the middle of the 15th century, in the large Spanish monasteries they used to locate the choir upon an elevated platform placed in the first sections of the main entrance to the temple. This platform should not be too high since the monks were to follow the religious ceremonies which took place in the main altar. These elevated choirs resulted in an extremely singular vault: the flat ribbed vault. The vault under the Choir of saint Esteban monastery in Salamanca is one of the most outstanding examples (photo 1).

The construction of this vault started with a collection of data that allowed us to carry out an accurate design, in plan and in elevation, of the vault. This drawing immediately reveals one of the most relevant features of the vault. While in the shorter section the ridge line (rampante) is a flat and horizontal line, in the perpendicular direction, it is slightly curved. These two lines are essential to establish the design and shape of the vault because they define the height of the boss stones situated upon both axes (photo 2).

The second most relevant feature in this vault concerns the ribs: all of them are extremely depressed oval arches. As it is known, an oval arch is an arc composed of three parts: the central part, extremely flat in this case, and the two arcs in the ends, much smaller. Nevertheless, a closer look of the vault allowed us to discover an extremely important particular: the small arches in the end of the oval are not strictly arches but are part of the solid springing of the vault: the tas-de-charges. The height reached by these small arches determines the height of the tas-de-charges; therefore the vault is built only with the upper part of the oval ribs (photo 3). [....]

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